In the production of plastics, paints, clays, pharmaceuticals, foods etc., various materials have to be reduced to ever finer particle sizes and mixed together or dispersed in fluids. In the production of paper, for example, the paper web issuing from a paper machine is coated with a lime-water suspension which has a fineness of about 5 .mu.m. In the preparation of this coating substance ("slurry" hereinafter), broken limestone from quarries or mines is crushed ever more finely down to the fineness referred to above.
Stirring bead mills serve for the production of fine products of this kind, especially slurries. Increasingly smaller grinding beads are used, as the fineness increases, so as to reduce the areas of contact between two grinding beads and particles that are still too coarse, thereby intensifying the grinding action. But the smaller the beads are the more difficult it is to hold them back in the grinding tank, so that only the sufficiently fine time particles will enter together with the suspending water into the fine material or end product, but not the grinding beads. This difficulty becomes still greater as the grinding beads become worn ever smaller in the course of operation.
During the development of the present invention it was learned that the grinding action is improved when the grinding beads do not have more or less the same size but when beads of different sizes operate together in the mill, for this increases the probability that the limestone particles of various sizes will be ground between grinding beads of matching size, i.e., larger particles will be ground between larger grinding beads and smaller particles between smaller grinding beads; it is important too that increasingly fine grinding beads be present as the fineness of the limestone particles increases.
At the same time it is to be considered that in the grinding process the grinding beads become ever smaller due to wear. While the mill operates, grinding beads of appropriate larger initial diameter can be added--most practically together with the coarse dispersion that is to be comminuted--and will become continually smaller as they operate within the mill, resulting in a range of sizes from large through medium to small and minimum size grinding beads. Initially, however, a mixture of grinding beads of varying sizes can be charged into the mill.
In particular it was recognized that the best grinding action is achieved when the grinding beads are completely worn away in the mill, i.e., when the grinding beads are retained in the mill until they have reached the maximum grain size of the fine suspension desired, i.e., of the end product. With the mills of the prior art this can be achieved only with difficulty.
This is because usually sieves for straining out the grinding beads are disposed at the discharge end of the grinding tank, between the grinding chamber and the fine product outlet. The sieve holes thus determine the size of the particles in the fine product: larger particles are held out by the sieve, finer ones pass through it. The finest sieves that can be used practically have sieve holes of about 100 .mu.m. This means that the fineness of the lime particles as well as that of the grinding beads that pass through amounts to about 100 .mu.m. This grain size is too large for a great many applications: a finer grain size of under 40 .mu.m is sought.
In the course of operation, however, the sieve holes and the sieve itself as a whole become clogged. A blanket then forms on the sieve and acts as a filter. This means that a pressure loss builds up as this process of forming a blanket on the sieve and clogging it progresses. Consequently the throughput, i.e., the amount of finished product produced per unit time, decreases. Therefore the sieve has to be reverse-flushed very often in order to remove the blanket or filter cake, resulting in down time and loss of production.
On account of the difficulties involved, attempts were made decades ago to hold out the grinding beads without sieves or filters.
In U.S. Pat. No. 3,653,600, a stirring bead mill was disclosed having a separator formed essentially by a cylindrical ring which is provided with a series of more or less radial bores uniformly distributed over the circumference. At the bottom end of this radially perforated ring a hub is formed, by which the separator is fastened on the shaft. Between the upper margin of the separator and the top cover of the mill a gasket is placed.
By means of such a separator the grinding beads are held back in the grinding chamber only by centrifugal force, so that sieves or filters with their above-mentioned problems could be avoided. However, this separator has found no acceptance in practice.
This might be due to the fact that the radial bores, despite their great number, generally offer a much too small cross section for the fine suspension flowing to them. In each of the many, relatively narrow bores the cross section is accordingly small, so that the velocity of flow is accordingly high, so that the centrifugal force cannot produce its effect. In this case it should be considered that centrifugal force increases with the square of the radius, i.e., is greatest radially outward and decreases radially inward by the square. Accordingly, the retention can take place substantially at the outer circumference alone, where the centrifugal force is correspondingly greatest: once particles have been drawn into a radial bore they are immediately subjected to a centrifugal force decreasing by the square, while the force of the flow is high in accord with the narrow cross section. Once particles are captured in a radial bore they no longer have any chance to be thrown outwardly. This is all the more true if, according to the embodiments of this patent, the separator body is surrounded by a sieve or filter, i.e., each radial bore is covered by a filter through which a particle--if it could at all penetrate radially into the bore--would have to be flung back out again by the sieve.
Other previously known attempts to create a bead retention without a sieve or filter consist in the fact that a plurality of plates at a small distance apart from one another are provided between the grinding chamber and the outlet chamber, forming gaps between them. The width of the gaps is smaller than the beads that are to be held out; smaller beads can flow through the gaps together with the fine product. However, even if a great number of such plates or gaps are used, the flow cross section is too small, so that problems are encountered in operation.